These systems are intended to project toward the eye of the user an aerial image collimated to infinity and superposed on the exterior landscape. The projected image may include a symbology representative of information on tasks, the mission to be accomplished or the exterior environment and/or even the image given by a video camera. The latter may be a low-light video camera or an infrared video camera or a thermal video camera.
The systems are either monocular or binocular depending on whether the user perceives the image with one eye or with both eyes.
These systems generally include a small high-resolution display on which the image to be projected is displayed, an optical relay and an optical combiner or “mixer” that ensures both the reflection of the image projected and collimated on the eye of the user and the transmission of the exterior landscape. This mixer is integrated into a screen or visor or a pair of glasses. By visor or screen, what is meant is a transparent curved element placed in front of the eyes of the user and the first function of which is to protect the eyes of the user while ensuring he has a good visibility of the exterior. This protection is mechanical but may also be optical if the visor includes particular treatments. In the rest of the text, the terms “screen” and “visor” will be used interchangeably to designate the support of the mixer.
In most applications, the visual field of the viewing system must be large, i.e. comprised between 20 degrees and 40 degrees. However, in most of the proposed solutions, the mixer is an optical element with a significant off-axis so as to ensure a good transmission and to leave the fewest possible optical elements in the field of view. Constructing a good-quality optical solution with a substantial off-axis is a first difficulty.
A second difficulty with this type of viewing system is that the integration of the mixer into the screen must disrupt as little as possible the vision of the exterior, human vision being naturally large-field. Thus, the human field of view reaches 200 degrees in a lateral plane and 125 degrees in a vertical plane. The mixer must therefore introduce the fewest possible visual obstructions and must have a shape that follows as closely as possible the contours of the face. It is also necessary for the system to be compatible with the wearing of with prescription eyeglasses.
Lastly, the weight and bulk of the system must remain small in order to allow it to be easily integrated under the forehead-facing portion of a helmet or to allow it to be placed directly on the face in the form of a pair of goggles, for example ski goggles or protective goggles, or even using a suitable skull cap or headband.
Mass-market viewing systems mounted on spectacle temples do not meet all these conditions.
Various technical solutions have been proposed. A first type of solution consists in placing the entire system laterally, the entire system then being contained in an essentially horizontal plane. The display and the optical relay are located on one of the lateral sides of the head of the user. By way of example, mention may be made of the devices described in patent applications WO 2010/089495 (entitled “Portable augmented-reality head-up display device”) and WO 2009/136393 (entitled “Wide angle helmet mounted display system”). These solutions have the drawbacks of being complex, bulky and of forming a lateral obstruction that may be significant.
A second type of solution consists in placing the entire system facing the forehead, the entire system being contained in a substantially vertical plane above the eye of the observer. By way of example, mention may be made of the devices described in patent applications WO 2013/036888 (entitled “Night vision devices and methods”) and U.S. Pat. No. 5,341,242 (entitled “Helmet mounted display”). As may be seen from the various figures of these patent applications, the optical solutions provided are complex. Their integration into the forehead-facing portion of a helmet is not simple. Moreover, they are essentially monocular solutions and the conversion of these solutions to binocular versions is problematic.
The head-borne viewing system according to the invention does not have these drawbacks. It may be monocular or binocular. It includes what are called crossed optics. By this what is meant, in the case of a binocular system, is that if one optical combiner is placed in front of the right eye of the user, the optical relay and the corresponding display are placed in a forehead-facing position above the left eye of the user and under the optical combiner located on the left-hand side. This arrangement is obtained by judiciously choosing the geometric parameters of the various optical elements, their curvatures and the form of their surfaces. This optical architecture has many advantages both as regards simplicity of construction and as regards bulk, and is easily accommodated under the forehead-facing portion of a helmet.